Topic 1.4: Mass Percent and Mixture Composition Quiz

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Boost your AP Chemistry score with this Topic 1.4 practice quiz. Test your understanding of mixture composition, mass percent, and elemental analysis for exam success.

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Mass Percent and Mixture Composition Quiz

17 MCQs

Topics Covered: Composition of Mixtures, Mass Percent, Elemental Analysis, Mixture Purity

Description: This Topic 1.4 quiz tests your understanding of mixture composition. You will practice determining the mass percent of specific elements and assessing the purity of a substance based on elemental analysis.

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1. A 2.50 g sample of an impure iron ore, primarily Fe₂O₃ (molar mass 159.7 g/mol), is reduced completely to solid iron (Fe). The mass of the pure iron recovered is 1.40 g. Assuming the impurities do not contain iron, what is the mass percent of Fe₂O₃ in the original ore sample?

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2. A sample of an unknown alkali metal chloride, MCl, is analyzed. The mass percent of chlorine in the sample is found to be 60.6%. Which of the following is the identity of the alkali metal, M?

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3. A mixture consists of two gases, He and Ne, in equal masses. Which of the following statements about this mixture is true?

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4. An impure sample of KClO₃ (molar mass 122.5 g/mol) is decomposed by heating, producing KCl and O₂ gas. A 10.0 g sample of the impure mixture produces 1.92 g of O₂. If the impurity does not decompose, what is the approximate purity (mass percent) of the KClO₃ in the sample?

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5. A sample of C₂H₄ (molar mass 28 g/mol) is contaminated with an unknown impurity. The mass percent of carbon in the impure sample is determined to be 89.5%. Pure C₂H₄ is 85.7% carbon by mass. Which of the following could be the impurity?

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6. The particulate-level diagram of a substance shows two different types of diatomic molecules, X₂ and Y₂, uniformly distributed in a container. This representation best describes:

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7. A 100 g sample of an alloy contains 80 g of Gold (Au) and 20 g of Silver (Ag). What is the mole fraction of Silver in the alloy? (Molar mass of Au = 197 g/mol, Ag = 108 g/mol)

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8. A student is given a mixture containing only solid Cu and solid Zn (brass). The student reacts a 2.00 g sample of the mixture with excess HCl(aq), which reacts completely with the Zn but leaves the Cu unreacted. If 1.30 g of unreacted Cu is recovered, what was the mass percent of Zn in the original sample?

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9. A pure sample of solid NaCl (molar mass 58.44 g/mol) contains 39.3% sodium by mass. A student analyzes a second, impure sample of NaCl and finds the mass percent of sodium to be 32.1%. Which of the following could be the identity of the impurity in the second sample?

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10. A 10.0 g sample of a mixture of sand (SiO₂) and salt (NaCl) is dissolved in water, filtered, and evaporated to dryness. If the dried salt has a mass of 3.5 g, what is the mass percent of sand in the original mixture?

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11. Two unlabelled white powders are known to be pure BaCl₂ and a mixture of BaCl₂ and NaCl. Elemental analysis of the pure BaCl₂ shows it is 65.9% barium by mass. The unlabelled mixture will have a barium mass percent that is:

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12. Magnesium reacts with oxygen to form MgO. A student is analyzing a sample of powdered magnesium that has been partially oxidized in the air, creating a mixture of Mg and MgO. The mass percent of oxygen in the mixture is found to be 20.0%. The mass percent of oxygen in pure MgO is approximately 40.0%. What is the mass percent of pure Mg in the mixture?

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13. A 5.00 g sample of a mixture containing only CaCO₃ (molar mass 100 g/mol) and an inert, non-reactive impurity is heated to decompose the CaCO₃ into CaO and CO₂ gas. If the mass of the solid sample decreases by 1.10 g after heating, what is the mass percent of CaCO₃ in the original mixture?

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14. A mixture of NO gas and NO₂ gas is placed in a rigid container. The mass percent of nitrogen in pure NO is 46.7%, and the mass percent of nitrogen in pure NO₂ is 30.4%. Elemental analysis of the gas mixture reveals that it is 40.0% nitrogen by mass. Which gas is present in a greater mass in the mixture?

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15. A student mixes 10.0 g of NaCl (39.3% Na by mass) and 10.0 g of KCl (52.4% K by mass). What is the approximate mass percent of chloride (Cl) in the total 20.0 g mixture?

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16. In a laboratory experiment, a student determines the mass percent of silver in a sample of AgCl by precipitating it from a solution of an unknown mixture. The student forgets to completely dry the AgCl precipitate before determining its mass. How will this error affect the calculated mass percent of silver in the original mixture?

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17. Two pure compounds, CO and CO₂, are mixed together. The mass percent of carbon in pure CO is 42.9%, and in pure CO₂ is 27.3%. If a mixture of the two gases is found to be 30.0% carbon by mass, what can be concluded about the composition of the mixture?

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The Pre-Quiz Review: Mass Percent and Mixture Composition

Before starting the quiz, make sure you can quickly identify what each mass, percentage, and chemical formula represents. The questions in this topic mainly test four skills: describing the composition of a mixture, calculating mass percent, using elemental analysis data, and determining the purity of a sample. The most important step is identifying the correct quantity before choosing an equation.

Composition of Mixtures: Identify the Total Sample and Each Component Before Calculating

A mixture contains two or more substances that are physically combined. Each substance is a component of the mixture. The total mass of the mixture is the sum of the masses of its components.

For a mixture containing A, B, and C:

massₜₒₜₐₗ = massₐ + massᵦ + mass𝒸

If one component is unknown:

massᵤₙₖₙₒ𝓌ₙ = massₜₒₜₐₗ − massₖₙₒ𝓌ₙ components

For AP Chemistry, carefully distinguish between the mass of the entire sample and the mass of one component. If a problem says a 40.0 g sample contains 12.0 g of NaCl, then 40.0 g is the total sample mass and 12.0 g is the NaCl mass.

If the mixture contains only two components:

mass percent A + mass percent B = 100%

For multiple components, their percentages add to 100% only when those components account for the entire sample.

Mass Percent: Know the Formula and, Most Importantly, the Correct Denominator

Mass percent tells you what percentage of the total sample’s mass is due to a particular component.

mass percent = (mass of component / mass of total sample) × 100

The numerator is the mass of the component being measured. The denominator is the mass of the entire mixture or sample.

Example:

A 50.0 g mixture contains 15.0 g NaCl.

mass percent NaCl = (15.0 g / 50.0 g) × 100 = 30.0%

The most common mistake is using the component mass as the denominator. Always ask: “Percentage of what?” The answer determines the denominator.

A mass percent of 30.0% means that 30.0 g of the component is present for every 100 g of the mixture, assuming the composition is represented by that percentage.

Converting Between Mass Percent, Component Mass, and Total Sample Mass

You should be able to rearrange the mass-percent relationship depending on what information is given.

If mass percent and total sample mass are known:

mass of component = (mass percent / 100) × massₜₒₜₐₗ

If component mass and mass percent are known:

massₜₒₜₐₗ = mass of component / (mass percent / 100)

Example:

A 250.0 g sample is 12.0% CaCO₃ by mass.

mass CaCO₃ = (12.0 / 100)(250.0 g) = 30.0 g

Remember that 12.0% must be converted to 0.120 when used as a multiplication factor.

Common trap: using 12 instead of 0.12 produces an answer 100 times too large.

Missing Components and Percentages: Use Conservation of Mass or the 100% Total

If the total mass of a mixture and the masses of some components are known, find the missing mass by subtraction.

For example, if a 100.0 g mixture contains 35.0 g A and 25.0 g B:

mass C = 100.0 g − 35.0 g − 25.0 g = 40.0 g

The same idea works with percentages. If a mixture contains only A, B, and C:

mass percent C = 100% − mass percent A − mass percent B

Use the 100% relationship only when the listed components account for the complete mixture. If a problem discusses only selected components, do not automatically assume that their percentages must add to 100%.

Elemental Analysis: Distinguish the Element From the Compound Containing It

Elemental analysis determines the amounts of individual elements present in a substance or sample. AP Chemistry questions may use elemental analysis to determine percent composition, empirical formulas, or the amount of a compound present in a mixture.

For a pure compound, the mass percent of an element is:

mass percent of element = (mass of element in 1 mol compound / molar mass of compound) × 100

For H₂O:

mass of H in 1 mol H₂O = 2(1.008 g) = 2.016 g

molar mass H₂O = 18.016 g/mol

mass percent H = (2.016 / 18.016) × 100 ≈ 11.19%

The chemical formula tells you how many moles of each element are present in one mole of the compound. Use the subscripts in the formula when determining the mass contribution of each element.

Elemental Analysis and Empirical Formulas: Convert Masses to Moles First

If a problem gives the masses of different elements and asks for an empirical formula, do not compare the masses directly.

Use:

moles = mass / molar mass

Then:

  1. Convert the mass of each element to moles.
  2. Divide every mole value by the smallest mole value.
  3. Look for a simple whole-number ratio.
  4. If necessary, multiply all ratios by the same small integer.
  5. Use the resulting whole-number ratios as the empirical-formula subscripts.

The key idea is that an empirical formula represents the simplest whole-number ratio of atoms, not the ratio of their masses.

For example, 12 g of carbon and 16 g of oxygen do not represent a 12:16 atom ratio. The masses must first be converted to moles.

Elemental Composition vs. Mixture Composition: Do Not Treat Them as the Same Quantity

This distinction is especially important in AP Chemistry problems.

Mixture composition asks:

“What fraction of the sample is a particular substance?”

Elemental composition asks:

“What fraction of a particular compound’s mass is a particular element?”

For example, if a sample contains CaCO₃, the mass of calcium is only part of the mass of CaCO₃. The mass of Ca cannot be substituted directly for the mass of CaCO₃.

If the fraction of an element in a compound is known:

mass of element = mass of compound × mass fraction of element

Therefore:

mass of compound = mass of element / mass fraction of element

This relationship can be used when elemental analysis is combined with mixture-composition questions.

Mixture Purity: Determine the Actual Mass of the Desired Substance

Purity tells you what fraction of an impure sample is actually the desired substance.

percent purity = (mass of pure substance / mass of impure sample) × 100

Therefore:

mass of pure substance = (purity / 100) × mass of impure sample

Example:

A 25.0 g sample is 80.0% pure Na₂CO₃.

mass Na₂CO₃ = (0.800)(25.0 g) = 20.0 g

Only 20.0 g should be treated as Na₂CO₃ in a calculation unless the problem provides additional information about the impurities.

The remaining 5.0 g represents impurities.

The denominator in a purity calculation is always the total mass of the impure sample.

Purity and Stoichiometry: Correct the Reactant Mass Before Converting to Moles

This is a high-value AP Chemistry skill. If an impure substance is used as a reactant, the entire sample mass is not the mass of the reacting substance.

Use this sequence:

impure sample mass → pure reactant mass → moles of reactant → mole ratio → requested quantity

Example:

A 40.0 g sample is 75.0% pure reactant.

mass of pure reactant = (0.750)(40.0 g) = 30.0 g

Then calculate:

moles of reactant = 30.0 g / molar mass

Only the 30.0 g of pure reactant should enter the stoichiometric calculation.

Do not convert the original 40.0 g directly into moles of the desired reactant. The remaining 10.0 g represents material that is not the desired reactant.

Purity, Mass Percent, and Elemental Percent: Similar Equations, Different Meanings

These quantities have similar mathematical structures but describe different things.

Mass percent of a component:

mass of component / total mixture mass × 100

Percent purity:

mass of pure desired substance / total impure sample mass × 100

Elemental mass percent:

mass of element in compound / mass of compound × 100

The calculation may look similar, but the quantities in the numerator and denominator change according to the question.

Before calculating, identify whether the question is asking about a component of a mixture, an element within a compound, or the desired substance within an impure sample.

AP Chemistry Problem Strategy: Identify the Quantity Before Choosing the Calculation

For any problem in this topic, first label the important quantities.

Ask:

  • What is the total sample mass?
  • What is the component mass?
  • Is the sample pure or impure?
  • Is the given mass an element or a compound?
  • Is the question asking for a percentage, mass, moles, or product?
  • Is a chemical reaction involved?

If no reaction is involved, the problem may only require mass percent, conservation of mass, or composition relationships.

If a reaction is involved, balance the equation and use the pure reactant amount for stoichiometry.

A common AP Chemistry pathway is:

impure sample → pure substance → moles → stoichiometric ratio → final quantity

Another common pathway is:

elemental mass → moles of element → empirical ratio → empirical formula

Do not choose an equation simply because the numbers look usable. First identify what each number physically represents.

Common Mistakes to Check Before You Submit an Answer

Before submitting an answer, check for the mistakes that most often occur in this topic.

Using component mass as the denominator in a mass-percent calculation.

Forgetting to divide a percentage by 100 before using it as a decimal.

Using the total impure sample mass as the mass of a reactant.

Treating the mass of an element as the mass of the compound containing that element.

Comparing elemental masses directly instead of converting them to moles for an empirical formula.

Assuming that listed components represent the entire mixture when the problem does not state this.

Forgetting that percentages representing all components of a sample must total 100%.

Using an unbalanced chemical equation for stoichiometry.

Rounding too early and allowing the rounding error to affect the final answer.

Forgetting units or reporting a percentage as a decimal when the question asks for a percent.

Final Quiz Checklist: Make Sure You Can Perform These Skills Without Notes

Before starting the quiz, you should be able to do each of these without needing to look up the relationship:

  • Calculate mass percent from component mass and total sample mass.
  • Calculate component mass from mass percent and total sample mass.
  • Calculate total sample mass from component mass and mass percent.
  • Determine a missing component using conservation of mass.
  • Determine a missing percentage when all components are accounted for.
  • Calculate elemental mass percent from a chemical formula.
  • Convert elemental masses to moles.
  • Determine an empirical formula from elemental-analysis data.
  • Distinguish elemental composition from mixture composition.
  • Determine the mass of a compound from the measured mass of one element in that compound.
  • Calculate percent purity.
  • Calculate the mass of pure substance in an impure sample.
  • Correct an impure reactant before performing stoichiometry.
  • Distinguish mass percent, elemental percent composition, and percent purity.
  • Use the correct denominator in every percentage calculation.
  • Track units and significant figures through the calculation.

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